Tuesday, 9 August 2011 was Census night in Australia. All people present in Australia on this night, with the exception of foreign diplomats and their families, should have been included on a Census form at the place where they stayed.

The Census of Population and Housing is the largest statistical collection undertaken by the Australian Bureau of Statistics (ABS) and one of the most important. Its objective is to accurately measure the number of people in Australia on Census night, their characteristics and the dwellings in which they live. Due to its size and complexity, whenever a Census is conducted it is inevitable that some people will be missed and some will be counted more than once.

It is for this reason that the Census Post Enumeration Survey (PES) is run shortly after each Census, to provide an independent measure of Census coverage. The PES determines how many people should have been counted in the Census, how many were missed, and how many were counted more than once. It also provides information on the characteristics of those in the population who have been missed or overcounted.

Some of the reasons why people may have been missed in the Census (i.e. undercounted) include:

they were travelling and were difficult to contact;

they mistakenly thought they were counted elsewhere;

there was insufficient space on the Census form in the household where they were staying and they did not obtain additional forms;

the person completing the form thought that certain people, for example, young babies, the elderly or visitors, should not be included;

they did not wish to be included due to concerns about confidentiality or a more general reluctance to participate;

the dwelling in which they were located was missed because it was difficult to find (e.g. in a remote or non-residential area); and

the dwelling in which they were located was mistakenly classed as unoccupied.

Some of the reasons why people may have been counted more than once (i.e. overcounted) or in error include:

they were included on the Census form at the dwelling where they usually live, even though they stayed and were counted elsewhere on Census night; and

they were overseas on Census night and so should not have been counted at all, but were included on the Census form at the dwelling where they usually live.

WHAT IS NET UNDERCOUNT?

While every effort is made to eliminate these potential causes of error, some undercount and overcount will inevitably occur. As is usually the case in Australia, in the 2011 Census more people were missed than overcounted and so the Census count of the population is fewer than the true population. This difference is referred to as net undercount.

Net undercount for any category of person is the difference between the PES estimate of the number of people who should have been counted in the Census and the actual Census count (including imputed persons in non-responding Census dwellings).

KEY USES OF NET UNDERCOUNT

Net undercount is the primary measure of Census coverage, and as such, is used in the following ways:

to augment Census counts, in order to derive the most robust estimate of the resident population (ERP) for 30 June of the Census year;

to provide users with an assessment of the completeness of the Census counts, allowing them to take this into account when using Census information; and

to evaluate the effectiveness of Census collection procedures so improvements may be made for future Censuses.

Accurate estimates of the resident population are required for a wide range of uses including: the allocation of seats to states and territories in the House of Representatives of the Australian Parliament; the distribution of Commonwealth payments to states and territories; as well as demographic, social and economic studies.

The PES is designed to be an independent check of Census coverage. Therefore, it is critical that the statistical independence between the PES and the Census is effectively managed, to ensure the PES is a robust check on Census coverage.

There are two aspects to statistical independence, both of which were effectively managed throughout the 2011 PES cycle: population independence and operational independence.

Population independence refers to the principle that there should be no sub-groups of the population where being missed in the Census indicates that a person or dwelling is also more likely to be missed by the PES. Although the PES estimation process adjusts for this to some extent, by subdividing the population into smaller groups where the assumption of population independence is more likely to be true, population independence is always more difficult to achieve than operational independence.

Selection to participate in the 2011 PES was based on a sample of private dwellings, meaning that those persons who were not living in, or visiting, a private dwelling at PES time were unavailable for selection. Therefore, although the PES has shown it is very effective in assessing overall Census coverage, its usefulness for estimating the undercount of certain sub-populations is limited, such as fly-in fly-out workers, who often live in non-private dwellings.

Operational independence requires that Census operations do not influence the PES in any way, and vice versa. The operational independence of the PES from the Census was effectively monitored at every stage of the 2011 cycle, including enumeration, processing and administration. Steps taken to ensure this independence included:

selecting the PES sample from an independent sample frame;

using separate office staff in the PES and Census;

ensuring PES interviewers were not employed as Census field staff in the same area;

maintaining the confidentiality of the PES sample so Census field and office staff were unaware of which areas were selected in the PES; and

ensuring Census forms received after PES enumeration commenced were excluded from PES estimation, thereby protecting the PES sample from having a higher proportion of Census response than in the overall population (due to contact from PES prompting respondents to return their Census forms).

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